The present invention relates to a ball screw apparatus of an end cap type for use in a feed apparatus and, in particular, to a ball screw apparatus which is suitable for high speed feed.
Conventionally, as a ball screw apparatus of this type, for example, there is known such a ball screw apparatus as shown in FIG. 10. This ball screw apparatus 1 comprises a screw shaft 3 and a ball nut 6 which is fitted with the screw shaft 3. The screw shaft 3 extends in the axial direction of the ball screw apparatus 1 and includes a screw groove 2; and, the screw groove 2 is defined in the outer peripheral surface of the screw shaft 3 and has a semicircular-shaped section. On the other hand, the ball nut 6 includes a screw groove 4 defined in the inner peripheral surface thereof and having a semicircular-shaped section. And, the ball nut 6 also includes a nut main body 6a to be fitted with the screw shaft 3, and an end cap 6b fixed to the axial-direction two end portions of the nut main body 6a through screws.
The screw groove 4 of the ball nut 6 and the screw groove 2 of the screw shaft 3 are disposed opposed to each other to thereby define a spiral-shaped passage between them; and, in the spiral-shaped passage, there are disposed a large number of steel-made balls 5 serving as rolling bodies in such a manner that they are allowed to roll along the spiral-shaped passage. And, due to the rotational movement of the screw shaft 3, the ball nut 6 is allowed to move linearly through the rolling movements of the balls 5.
By the way, when the ball nut 6 moves linearly, the balls 5 move while rolling along the spiral-shaped passage defined by the two screw grooves 2 and 4. However, in order to be able to move the ball nut 6 continuously, it is necessary to circulate the balls endlessly.
In view of this, not only, within the nut main body 6a, there is defined a ball circulation hole 8 which penetrates through the nut main body 6a in the axial direction thereof, but also, between the end face of the nut main body 6a and end cap 6b, there is defined a ball circulation R portion 7 which allows the ball circulation hole 8 and two screw grooves 2, 4 to communicate with each other; and, the ball circulation R portion 7 and ball circulation hole 8 cooperate together in defining a ball circulation passage 9 through which the balls 5 rollingly moving between the two screw grooves 2 and 4 are allowed to circulate endlessly.
In the ball circulation R portion 7, as shown in FIG. 11, a guide groove 10 defined in the end face of the nut main body 6a and a guide groove 11 defined in the end cap 6b are disposed opposed to each other to thereby define a circulation passage for the balls 5. Also, as shown in FIG. 12, on the end cap 6b, there is disposed a tongue 12 in such a manner that it projects from the screw groove 2 of the screw shaft 3; and, this tongue 12 is used to scoop up the balls 5 rollingly moving between the two screw grooves 2 and 4 in a load area and then guide the balls 5 to the ball circulation R portion 7.
Here, FIGS. 13 and 14 show the details of the conventional end cap 6b. As can be seen from FIGS. 13 and 14, in the end cap 6b, there is defined a concave-shaped guide groove 11 which defines a tongue 12 for scooping up the balls 5 and ball circulation R portion 7; that is, the end cap 6b has a three-dimensional complicated shape. Also, since it is difficult to work the tongue 12 and guide groove 11 at the same time, the tongue 12 and guide groove 11 are easy to differ in position from each other. Therefore, there is a possibility that this position difference can impede the smooth circulatory movements of the balls 5 to thereby degrade the operation of the ball screw apparatus and incur an increase in noises.
Also, in case where the end cap 6b is worked using a machine, the end cap 6b is high in cost due to the complicated shapes of the tongue 12 and guide groove 11. Therefore, generally, the end cap 6b is molded of not metal material but resin material. However, it is difficult to adapt the resin-molded end cap flexibly to various specifications of the shaft diameter and lead of the screw shaft 3 and the resin-molded end cap is poor in heat resistance, chemical resistance and strength against a slight collision.
Further, as described above, in the ball circulation R portion 7, the balls 5 rollingly moving between the two screw grooves 2 and 4 not only are scooped up in the load area but also are returned to the ball circulation hole 8, that is, the rollingly moving balls 5 are received and delivered on these two sides. However, as shown in FIG. 11, in the structure where the end faces of the end cap 6b and nut main body 6a respectively include the guide grooves 11 and 10 for the balls 5, in case where the working accuracy of the respective guide grooves 11 and 10 is not high, the guide grooves 11 and 10 cannot be matched to each other in position properly with no difference at the same time on the two ball reception and delivery sides. To avoid this, as shown in FIGS. 13 and 14, there are disposed, in the end faces of the end cap 6b and nut main body 6a, projections or recesses 13 for positioning the two guide grooves 11 and 10. However, in fact, whether the guide grooves 11 and 10 can be matched to each other in position properly with no difference at the same time on the two ball reception and delivery sides or not depends on the working accuracy of the guide grooves 11 and 10. Therefore, in case where this working accuracy is poor, the operation of the ball screw apparatus can be worsened and noises can be increased.